The present invention relates to a radio transmission device, a radio receiving device, a radio transmitting/receiving system, and a storage medium, which are used for transmitting a real time signal such as sound and image by means of a radio communication method. In particular, the present invention relates to a radio transmission device, a radio receiving device, a radio transmitting/receiving system, and a storage medium, which are used for transmitting a real time signal while keeping data quality of the signal by means of a radio communication method.
More specifically, the present invention relates to a radio transmission device, a radio receiving device, and a radio transmitting/receiving system, which are used for transmitting a real time signal such as sound and image in a radio transmission system where effective throughput fluctuates (that is to say, it is not guaranteed) like ACL link (Asynchronous Connection Less Link) of Bluetooth. In particular, the present invention relates to a radio transmission device, a radio receiving device, and a radio transmitting/receiving system, which are capable of transmitting a real time signal while keeping its data quality in a transmission system where effective throughput fluctuates like the ACL link of the Bluetooth.
Recently, short-distance radio communication technology is becoming highlighted. According to this kind of radio communication method, cordless communication between equipment and a terminal can be established. Therefore, connection work between pieces of equipment is simplified. In addition, because it is not necessary to choose an installation location for each equipment, this kind of radio communication method is very convenient. Short-distance radio communication is also highly expected to be a transmission medium, which is placed in a local location where construction of a network cable is not realistic like, for example, a home network, etc.
For example, the short-distance radio communication can be used for the following: data exchange between pieces of portable information equipment; and transmission of data and sound between a main body of a telephone (or a portable music player) and a headset, between a main phone and a cordless handset, and between others.
“Bluetooth”, which is a representative example of the short-distance radio communication, uses a radio frequency of 2.45 GHz band. Data transmission speed is 1 Mbps as a whole, in which a synchronous transmission channel of 64 kbps available for telephone voice transmission and an asynchronous transmission channel for data transmission are provided. The former synchronous transmission channel adopts SCO (Synchronous Connection Oriented Link) transmission method, and is applied to line connection. On the other hand, the latter asynchronous transmission channel adopts ACL (Asynchronous Connection Less Link) transmission method, and is applied to data transmission using packet switching.
A range of connection between pieces of equipment by the Bluetooth is about 10 m. However, the range can be extended up to 100 m using an additional amplifier. The Bluetooth is based on point-to-point connection between master equipment and slave equipment, or point-to-multipoint connection. In the latter case, one master can communicate with seven slaves at a maximum by means of the SCO or the ACL method.
For example, there may be an application that communicates between pieces of equipment for handling a real time signal such as sound and image (high fidelity audio, etc.) using the Bluetooth.
However, there is a high possibility that transmission throughput fluctuates on a radio transmission channel. Therefore, in order to transmit a real time signal without interruption of sound and image by means of the radio communication method, it is necessary to make enough allowance for line quality. As a result, maximum throughput of a line cannot be used effectively.
Concerning information communication, in general, on the transmission side, transmission data is encoded and compressed; and on the receiving side, received data is decoded (to be more specific, Codec processing is performed). In addition, a compression rate is changed in response to the line quality. However, in this case, the change is based on the concept that when allowance of throughput becomes small, the data compression rate is increased. Because of it, it is necessary to know the allowance of throughput.
At this point, the following case will be considered: a real time signal such as sound and image is transmitted using the short-distance radio communication such as the Bluetooth.
As described above, the Bluetooth has two kinds of transmission methods: the SCO and the ACL. If the SCO method based on line connection is used, no retransmission request is issued regardless of whether or not there is an error correction. Therefore, sound quality and image quality are not guaranteed. In contrast to this, the ACL is a packet transmission method. When an error occurs (or when the error is not corrected by error correction), an error-free state is realized by packet retransmission. Therefore, sound quality and image quality can be guaranteed. In other words, the ACL method can be said to be suitable for transmission of a real time signal because data quality is guaranteed, and because sound and image can be transmitted satisfactorily without interruption.
However, the ACL method is a link of Best Effort type. Therefore, as a trade-off for guarantee of data quality, a situation, in which latency and throughput cannot be guaranteed when line quality decreases, occurs (for example, when utilizing radio communication at home, operation of ordinary home electric appliances such as microwave oven causes effective throughput to change to a large extent).
In other words, the ACL link in the Bluetooth is a communication system in which effective throughput changes. There is no way of knowing a real-ability value of throughput. Therefore, a mechanism, which determines a data compression rate in response to allowance of throughput, does not hold.